1. Field of the Invention
The present invention relates in general to nuclear well logging, and pertains in particular to improved methods and apparatus for analyzing inelastic scattering gamma ray energy spectra to provide more accurate information of the composition of earth formations surrounding a well bore.
2. The Prior Art
Heretofore, various techniques have been utilized to process gamma ray energy spectra for borehole constituent analysis. In the case of inelastic scattering gamma ray energy spectra, it is known that analysis of the spectra to identify the contributions thereto due to carbon and oxygen provides useful information of the presence of oil in a formation. Additional information concerning the composition of the formation, such as its lithology for instance, is however frequently required before an unambiguous determination of the presence of oil can be made. A suitable lithology indicator for this purpose might comprise the ratio of inelastic scattering gamma ray contributions for calcium and silicon.
The derivation of the foregoing information concerning carbon, oxygen, calcium and silicon, and possibly other constituents of the formation and well bore, depends upon accurate constituent analysis of the formation gamma ray spectra. An important and basic technique for performing such analysis is disclosed in U.S. Pat. No. 3,521,064, issued on July 21, 1970 to Moran et al. In accordance with the Moran et al. teaching, a detected gamma ray energy spectrum for a formation of unknown composition is compared with a composite spectrum made up of weighted standard spectra of the constituents postulated to comprise the formation. The weight coefficients for the standard spectra which give the best fit of the composite spectrum to the unknown spectrum, as determined, for example, by the method of least squares, represent the relative proportions of the constituents in the formation. By appropriate selection of the standard spectra, the proportions of the constituents of interest, such as carbon, oxygen, calcium, silicon, etc., may be obtained, from which the desired information regarding oil content may be derived.
Although the Moran et al. technique, as disclosed in U.S. Pat. No. 3,521,064, is applicable for the purpose of the present invention and in this respect provides substantial advantages relative to other prior art techniques, the present invention is concerned with methods and apparatus which provide still better results, particularly in connection with the analysis of inelastic scattering gamma ray spectra.
To obtain statistically accurate inelastic scattering gamma ray spectra it is desirable to irradiate the formation with neutrons at as high a repetition rate as is practicable. Closely spaced neutron bursts have the disadvantage that background gamma rays, resulting in this instance predominately from thermal neutron capture reactions between formation constituents and neutrons from one or more preceding bursts, will be present during the detection periods for the inelastic scattering gamma rays. Such capture gamma rays will of course be sensed by the detector and, unless compensated for, will tend to degrade the inelastic scattering gamma ray spectra.
Moran et al. suggest in U.S. Pat. No. 3,521,064 that the capture gamma ray component in the detected inelastic scattering gamma ray spectrum may be accounted for by previously generating a separate "background" spectrum representative of residual capture gamma radiation from prior bursts and including such spectrum as a standard in the composite spectrum. According to the Moran et al. patent, the standard background spectrum is taken beforehand in a reference borehole or test pit. This, however, does not necessarily reflect the real in situ capture gamma ray spectrum, which varies with change in neutron source strength, sonde environment, sonde performance, etc., and thus may lead to inaccuracies in the constituent proportions obtained from the spectrum matching process.
As evidenced by U.S. Pat. No. 3,780,303 to Smith et al., it has also been proposed in the prior art to detect the level of background gamma radiation immediately before each neutron burst, and then subtract that level from the inelastic scattering gamma ray counts obtained during the burst. Since the gamma rays observed during the background detection period result predominately from thermal neutron capture, the assumption is that the inelastic scattering gamma ray spectrum will be corrected in a proper way for the presence of capture gamma rays lingering from prior bursts. This, however, is not the case.
The Smith et al. background gamma ray count reflects only an approximation of the total background level prevailing during the succeeding neutron burst (assuming that the background detection period closely precedes the burst and that the inelastic scattering detection period is short relative to the thermal neutron decay time constant of the formation). It does not afford information of the spectral character or shape of the capture gamma ray spectrum and thus does not accurately compensate the inelastic gamma ray spectrum for the influence of residual capture gamma radiation from prior neutron bursts.